I'm having some soundproofing added to a party wall (1930's semi) and was advised to first seal any gaps around the joists to stop sound getting through there. I've got the floorboards up and sure enough there are big gaps around the joists, in some places around a brick width deep and 3/4" high. I read that I should use either "cement" or mastik to fill the gaps. Two questions: 1) Which would be best? I'm going to need a lot of mastik... 2) Do they mean cement or mortar - is there any reason why I wouldn't use mortar like the rest of the wall?
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Sealing around joists
- Thread starter mark482
- Start date
expanding foam and mortar. 
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